Improve "isNullOr"

[dimsuz]

Switch from

fun <I : Any, E> Validator.Companion.isNullOr(validator: Validator<I, I, E>): Validator<I?, I?, E>

to

fun <I : Any, O : Any, E> Validator.Companion.isNullOr(validator: Validator<I, O, E>): Validator<I?, O?, E>